The response of breast cancer to hormonal therapy is strongly influenced by the level of estrogen receptor (ER) expression in the tumor. Currently, measures of the ER content of the primary tumor are used for breast cancer treatment selection. This approach is limited by heterogeneity of ER expression within tumors and between tumor sites. Up to 50% of patients with ER-positive tumors fail to respond to hormonal therapy. Furthermore, almost all patients initially responding to a particular hormonal agent will eventually fail therapy; but we have little insight to the timing of this failure or the likelihood of responding to alternative hormonal agents. The overall goal of this project is to measure regional tumor ER content in vivo. We hypothesize that we can predict response to hormonal therapy by non-invasive assessment of tumor estrogen binding using PET imaging and [F18]-16 alpha-fluoroestradiol (FES). Initial studies aimed at formulating quantitative imaging protocols in a variety of breast cancer patients have been completed. We have tested methods of quantifying estrogen binding in tumors using FES and have found good correlation between measures of FES uptake and quantitative ER immunocytochemistry in preliminary patient studies. This methodology will be applied to three aims: (1) Validate the ability the FES to measure ER content in both primary and metastatic tumors compared against assay of biopsy specimens, (2) Measure the heterogeneity of ER expression in patients with large tumors and/or multiple lesions using FES imaging and test the ability of these measurements to predict early response to hormonal therapy, and (3) Monitor the degree of estrogen blockade and changes in receptor expression over the course of tamoxifen therapy to identify potential mechanisms of acquired resistance. In addition to patient studies, we will perform laboratory and animal studies of FES transport and uptake into tissue to refine our analysis of FES PET images. We will measure FES binding to sex steroid binding protein (SBP) and FES and metabolite uptake into ER- containing tissue. In summary, we will use PET estrogen receptor imaging to answer basic question regarding the biology of the ER in breast cancer. This will lead to significant improvements in the use of hormonal therapy in breast cancer, including better selection of patients for hormonal therapy and better decisions on when and how the change therapy in those patients who fail tamoxifen therapy.